build: Allow clang build linkage to use libcompiler-rt

Make use of '-print-librt-file-name' over '-print-libgcc-file-name'
to use Compiler-RT runtime glue over libgcc glue.

NOTE: *** Requires at least clang 3.6.x

Change-Id: I7f63284473d6067bf775409970c8dd98f5d5a8d5
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/6144
Reviewed-by: Marc Jones <marc.jones@se-eng.com>
Tested-by: build bot (Jenkins)
3 files changed